The present invention relates to devices for transferring power from a rotating input shaft to an output shaft. In particular, the present invention relates to torque converters, automatic gearboxes, and power transfer cases which employ clutches for effecting the transfer of power.
Over the past several years, automotive manufacturers have been forced to produce automobiles which are more fuel efficient and produce less harmful emissions. The vast majority of improvements in this area have been with fuel delivery systems, emission control systems and vehicle aerodynamics. However, torque converters, automatic gearboxes and power transfer cases are other, often overlooked, sources of inefficiency and harmful emissions.
Conventional torque converters consist of an impeller coupled to a rotating input shaft, a turbine coupled to an output shaft, and a stator in communication with the impeller and the turbine. As the impeller blades rotate, the impeller directs hydraulic fluid between the stator and the turbine, urging the stator to rotate in the direction opposite to that of the impeller and the turbine. However, in order that the input torque from the impeller is amplified, the stator must be prevented from reverse rotation. On the other hand, during high speed operation, it is desirable to couple the impeller directly to the turbine through a lock-up disc clutch. In this mode, commonly referred to as converter lock-up, it is necessary for the stator to rotate with the impeller and the turbine and in the same direction as the impeller and the turbine. Accordingly, it has been common practice to couple the stator to the converter housing through a roller ramp one-way clutch which prevents the stator from rotating backwards, but allows the stator to rotate with the impeller and the turbine during converter lock-up.
However, when an automobile fitted with a conventional torque converter is stopped, with the brakes engaged and the engine still running, the torque converter subjects the engine to drag since the turbine is no longer free to move. As a consequence, it is necessary to idle the engine at increased speed to overcome the drag placed on the engine. The increased idle speed requires more fuel to be consumed and produces environmentally harmful emissions. Furthermore, the roller ramp one-way clutch does not rotate freely even when freewheeling. As a result, when the torque converter is in lock-up mode and the stator rotates with the impeller and turbine, the one-way clutch again produces a small drag on the engine thereby increasing fuel consumption.
Attempts have been made to improve the conventional torque converter. For instance, Ferodo (FR-A-1 480 241) teaches a torque converter which comprises an impeller coupled to a bell housing, a turbine coupled to an output shaft, a stator disposed between the impeller and the turbine, and a clutch provided between the bell housing and the turbine for facilitating converter lock-up. The stator is rotatably disposed around the output shaft and is coupled to a one-way clutch for reducing drag at converter lockup. However, the torque converter taught by Ferodo cannot be used to address the problem of engine drag and hydrocarbon emission when the output shaft is stationary.
Welch (U.S. Pat. No. 3,724,208) teaches a friction device for a torque converter, comprising a conical friction member splined to a grounded sleeve, and an axially-movable conical friction member disposed around the grounded friction member and secured to the stator. In operation, the turbine delivers hydraulic fluid into the turbine, which then directs the fluid onto the front faces of the stator blades. The resultant axial force exerted by the fluid on the stator blades drives the movable friction member into engagement with the grounded friction member, thereby locking the stator and allowing the converter to multiply input torque. As the turbine accelerates, the angle of attack of the fluid leaving the turbine diminishes until the fluid is directed onto the rear faces of the stator blades, causing the axial force on the moveable friction member to be reduced to zero. At this stage, the moveable friction member is released from the grounded friction member and the converter fictions as a fluid coupling. However, as with Ferodo, the torque converter taught by Welch cannot be used to address the problem of engine drag and hydrocarbon emission when the output shaft is stationary.
Conventional automatic gearboxes consist of a planetary gear set comprising a torque input member coupled to an input shaft, a torque output member coupled to an output shaft, and a reaction member in communication with the torque input member and the torque output member. To effect gear changes, the automatic gearbox includes a first clutch pack for grounding the reaction member to the gearbox casing, and a second clutch pack for coupling the reaction member either to the torque input member or the torque output member. However, the magnitude of the torque required to be transmitted by the clutch packs has led to the use of expensive disc clutches having a large number of plates, thereby increasing the weight of the gearbox. Further, since each clutch pack has only two states, namely engaged or disengaged, shifting between gear is often abrupt. These abrupt changes detrimentally affect fuel flow, thereby reducing fuel efficiency and increasing fuel consumption.
Conventional four-wheel drive power transfer cases consist of an input shaft, a rear wheel torque output shaft coupled to the input shaft, a front wheel torque output shaft; and a torque transfer gear assembly for transferring torque from the input shaft to the front wheel torque output shaft. The torque transfer assembly generally consists of a first sprocket splined to the input shaft, a second sprocket rotatably disposed around the front wheel torque output shaft, a chain trained around the first and second sprockets, a series of splines provided on the front wheel torque output shaft, and a splined ring for coupling the second sprocket to the front wheel torque output shaft. To reconcile the differential rotation between the front and rear torque output shaft during vehicle manouevres, manufacturers generally include a friction clutch disposed between the second sprocket and a coaxially mounted shaft which allows clutch slippage to occur. However, clutch slippage interrupts smooth power transfer during cornering, thereby reducing fuel efficiency and fuel consumption.
Accordingly, it would be environmentally beneficial to provide torque converter designs, automatic gearbox designs and power transfer case designs which reduced the inefficiencies associated with the use of the clutches employed therein.
It is an object of the invention to provide a power transfer device which reduces the inefficiencies associated with the prior art power transfer devices.
According to a first embodiment of the invention, there is provided a power transfer device comprising a torque input member; a torque output member; a torque reactive member coupled to a rotational reference and being in communication with the torque input member and the torque output member for amplifying and transmitting torque from the torque input member to the torque output member; and a clutch responsive to torque input member rotational speed for releasing the torque reactive member from the rotational reference when the rotational speed reaches a threshold value.
According to a second embodiment of the invention, there is provided a power transfer device comprising a torque input member; a torque output member; a reactive member in communication with the torque input member and the torque output member; and a binary clutch for automatically switching the reactive member between a first mode coupling to a rotational reference and a second mode coupling to one of the torque input member and the torque output member. The binary clutch comprises a first clutch for coupling the reactive member to the torque member, and a one-way clutch for progressively coupling and uncoupling the reactive member to and from the rotational reference when switching between the first and second mode.
According to a third embodiment of the invention, there is provided a power transfer device comprising a torque input member; a rear wheel torque output member coupled to the torque input member; a front wheel torque output member; and a torque transfer assembly for transferring torque from the torque input member to the front wheel torque output member. The torque transfer assembly comprises a pair of one-way clutches coupled between the torque input member and the front wheel torque output member and having opposite freewheeling directions of rotation for allowing overrunning of the front wheel torque output member independently of the direction of rotation of the torque input member.